CNIT 127: Exploit Development

Ch 1: Before you begin

Basic Concepts

Vulnerability

• A flaw in a system that allows an attacker to do something the designer did not intend, such as– Denial of service (loss of availabiltiy)– Elevating privileges (e.g. user to Administrator)– Remote Code Execution (typically a remote shell)

Exploit

• Exploit (v.)– To take advantage of a vulnerability and cause a

result the designer did not intend• Exploit (n.)

– The code that is used to take advantage of a vulnerability

– Also called a Proof of Concept (PoC)

0Day and Fuzzer

• 0Day– An exploit that has not been publicly disclosed– Sometimes used to refer to the vulnerability itself

• Fuzzer– A tool that sends a large range of unexpected

input values to a system– The purpose is to find bugs which could later be

exploited

Memory Management

• Specifically for Intel 32-bit architecture• Most exploits we'll use involve overwriting or

overflowing one portion of memory into another

• Understanding memory management is therefore crucial

Instructions and Data

• There is no intrinsic difference between data and executable instructions– Although there are some defenses like Data

Execution Prevention• They are both just a series of bytes• This ambiguity makes system exploitation

possible

Program Address Space

• Include– Actual program instructions– Required data

• Three types of segments– .text contains program instructions (read-only)– .data contains static initialized global variables

(writable)– .bss contains uninitialized global variables

(writable)

Stack

• Last In First Out (LIFO)• Most recently pushed data is the first popped• Ideal for storing transitory information

– Local variables– Information relating to function calls– Other information used to clean up the stack after

a function is called• Grows down

– As more data is added, it uses lower address values

Heap

• Holds dynamic variables• Roughly First In First Out (FIFO)• Grows up in address space

Program Layout in RAM

• From link Ch 1a (.bss = Block Started by Symbols)

Assembly

Assembly Language

• Different versions for each type of processor• x86 – 32-bit Intel (most common)• x64 – 64-bit Intel• SPARC, PowerPC, MIPS, ARM – others• Windows runs on x86 or x64• x64 machines can run x86 programs• Most malware is designed for x86

Instructions

• Mnemonic followed by operands• mov ecx 0x42

– Move into Extended C register the value 42 (hex)• mov ecx is 0xB9 in hexadecimal• The value 42 is 0x4200000000• In binary this instruction is• 0xB942000000

Endianness

• Big-Endian– Most significant byte first– 0x42 as a 64-bit value would be 0x00000042

• Little-Endian– Least significant byte first– 0x42 as a 64-bit value would be 0x42000000

• Network data uses big-endian• x86 programs use little-endian

IP Addresses

• 127.0.0.1, or in hex, 7F 00 00 01• Sent over the network as 0x7F000001• Stored in RAM as 0x0100007F

Operands

• Immediate– Fixed values like –x42

• Register– eax, ebx, ecx, and so on

• Memory address– Denoted with brackets, like [eax]

Registers

Registers

• General registers– Used by the CPU during execution

• Segment registers– Used to track sections of memory

• Status flags– Used to make decisions

• Instruction pointer– Address of next instruction to execute

Size of Registers

• General registers are all 32 bits in size– Can be referenced as either 32bits (edx) or 16 bits

(dx)• Four registers (eax, ebx, ecx, edx) can also be

referenced as 8-bit values– AL is lowest 8 bits– AH is higher 8 bits

General Registers

• Typically store data or memory addresses• Normally interchangeable• Some instructions reference specific registers

– Multiplication and division use EAX and EDX• Conventions

– Compilers use registers in consistent ways– EAX contains the return value for function calls

Flags

• EFLAGS is a status register• 32 bits in size• Each bit is a flag• SET (1) or Cleared (0)

Important Flags

• ZF Zero flag– Set when the result of an operation is zero

• CF Carry flag– Set when result is too large or small for destination

• SF Sign Flag– Set when result is negative, or when most significant

bit is set after arithmetic• TF Trap Flag

– Used for debugging—if set, processor executes only one instruction at a time

EIP (Extended Instruction Pointer)

• Contains the memory address of the next instruction to be executed

• If EIP contains wrong data, the CPU will fetch non-legitimate instructions and crash

• Buffer overflows target EIP

Simple Instructions

Simple Instructions

• mov destination, source– Moves data from one location to another

• Intel format is favored by Windows developers, with destination first

• Remember indirect addressing– [ebx] means the memory location pointed to by

EBX

lea (Load Effective Address)

• lea destination, source• lea eax, [ebx+8]

– Puts ebx + 8 into eax• Compare

– mov eax, [ebx+8]– Moves the data at location ebx+8 into eax

Arithmetic

• sub Subtracts• add Adds• inc Increments• dec Decrements• mul Multiplies• div Divides

NOP

• Does nothing• 0x90• Commonly used as a NOP Sled• Allows attackers to run code even if they are

imprecise about jumping to it

The Stack

• Memory for functions, local variables, and flow control

• Last in, First out• ESP (Extended Stack Pointer) – top of stack• EBP (Extended Base Pointer) – bottom of stack• PUSH puts data on the stack• POP takes data off the stack

Other Stack Instructions

• All used with functions– Call– Leave– Enter– Ret

Function Calls

• Small programs that do one thing and return, like printf()

• Prologue– Instructions at the start of a function that prepare

stack and registers for the function to use• Epilogue

– Instructions at the end of a end of a function that restore the stack and registers to their state before the function was called

Conditionals

• test– Compares two values the way AND does, but does

not alter them– test eax, eax

• Sets Zero Flag if eax is zero

• cmp eax, ebx– Sets Zero Flag if the arguments are equal

Branching

• jz loc– Jump to loc if the Zero Flag is set

• jnz loc– Jump to loc if the Zero Flag is cleared

C Main Method

• Every C program has a main() function• int main(int argc, char** argv)

– argc contains the number of arguments on the command line

– argv is a pointer to an array of names containing the arguments

Example

• cp foo bar• argc = 3• argv[0] = cp• argv[1] = foo• argv[2] = bar

Recognizing C Constructs in Assembly

Incrementing

Cint number;...number++;

Assemblernumber dw 0...mov eax, numberinc eaxmov number, eax

• dw: Define Word

Incrementing

Cint number;if (number<0){...}

Assemblernumber dw 0mov eax, numberor eax, eaxjge label...label :

• or compares numbers, like test (link Ch 1b)

Array

Cint array[4];...array[2]=9;

Assemblerarray dw 0,0,0,0...mov ebx, 2mov array[ebx], 9

Triangle

Cint triangle (int width, int height) {int array[5] = {0,1,2,3,4};int areaarea = width * height/2;return (area);}